This invention relates to an irradiation light beam tracking servo drawing device for an optical type information reading device.
In an optical type video disk, recesses called "pits" are arranged on the surface thereof to form concentric tracks or a spiral track. Video information is recorded by the lengths and intervals of these pits.
In reading information out of the video disk, a light beam is applied to the video track comprising these pits, and th reflection light beam modulated by the presence or absence of the pits is demodulated. The information reading device of such an information recording disk is provided with a so-caled "tracking servo device" so that the irradiation light beam is correctly applied to the video track at all times.
FIG. 1 is an explanatory diagram showing the outline of one example of a tracking servo device. A light beam from a light source 1 such as helium neon laser is passed through a collimeter lens 2, a beam splitter 3, .lambda./4 plate 4, a tangential mirror 5, a tracking mirror 6 and a focus lens 7, to be focused on the record surface of a video disk 8. The reflection light beam is applied through the focus lens 7, the tracking mirror 6 and the tangential mirror 5 to the .lambda./4 plate 4 and the beam splitter 3, where the incident light beam and the reflection light beam which has read the information are effectively separated.
The reflection light beam separated by the beam splitter 3 is applied to another beam splitter 11, where it is separated into a light beam for servo operation and a signal reproducing light beam. The signal reproducing light beam is detected by a light receiving element (not shown) and it is suitably processed to be reproduced as a signal. On the other hand, the light beam for servo operation is applied through a cylindrical lens 12 to the light receiving surface of a light receiving element 9 provided following the lens 12 in the light path.
The light receiving element 9 is designed to produce a tracking servo signal adapted to control the rotation of the tracking mirror 6 and a focus servo signal which is employed so that the vertical movement of the focus lens 7 is controlled with respect to the disk 8. Hence, the irradiation light beam is focused on the record surface at all times. More specifically, as shown in FIG. 2, the light receiving element 9 is made up of four light receiving units 9a, 9b, 9c and 9d. The light beam passing through the cylindrical lens 12 is focused on the two focal lines being in the plane including the generating lines of the cylindrical lens and in the plane perpendicular to the aforementioned plane.
By utilizing the principle, the configuration changes of the beam spot projected onto the four separate light receiving units of the light receiving element 9 are detected and measured to determine the positional relation between the focus lens 7 and the record surface. This is used to control the vertical movement of the lens 7, that is, the focus servo operation is carried out.
In the tracking servo operation, the sums of the outputs of the adjacent light receiving units 9a and 9d, and 9b and 9c, i.e. the sums (Va+Vd) and (Vb+Vc) are applied to a comparison amplifier 10, and the difference voltage Ve is used as an error signal for tracking servo.
When the center of the irradiation light beam is set on the center line of the video track by suitably setting the relation between the cylindrical lens 12, the light receiving element 9 and the video track, then the images of the pits are projected onto the light receiving surfaces as indicated by reference numeral 20 in FIG. 3(a). When the center of the irradiation light beam is shifted from the center line of the video track, then the images of the pits are projected onto the light receiving surfaces as indicated by reference numeral 20 in FIG. 3(b). That is, the output Ve of the comparison amplifier 10 is an error signal corresponding to a distance between the center of the light beam and the track center line. The tracking mirror 6 is turned by a necessary angle through using the error signal, so that the light beam is correctly on the track at all times.
The tangential mirror 5 is used to control the position of the light beam in the tangential direction of the video disk, and serves as a time base corrector. The detailed description of the tangential mirror 5 will be omitted since its function and construction are well known in the art.
Although the video tracks of the video disk are provided substantially concentric in general the center of those concentric circles is not coincident with the center of rotation of the video disk which is obtained when it is rotated on a reproducing device. Therefore, when the tracking servo loop is open, the reading irradiaton light beam spot goes obliquely across the tracks Y as indicated by the arrow X in FIG. 4(a). If, under this condition, the tracking servo loop is closed, the tracking mirror immediately carries out an abrupt operation to follow the track. This is especially true when the relative speed between the irradiation light beam and the track is high. Then, the tracking mirror causes the light beam to pass over the track. Thus, the mirror is vibrated until the servo is completely locked, and in a worst case it is impossible to lock the servo.